1. Field of the Invention
The present invention relates to a sheet feeding apparatus which is used for a scanner, facsimile, copier, or printer, for example, and feeds originals separating one by one.
2. Description of the Related Art
FIG. 10 is a view showing a scanner including a conventional sheet feeding apparatus.
The sheet feeding apparatus shown in FIG. 10 includes a pickup roller 102 that contacts the top surface of a batch of originals (sheets) stacked on a sheet supply tray 101 and feeds out the originals in a sheet feeding direction. The originals 100a picked up by the pickup roller 102 are separated one by one and fed to a conveying pass by a feed roller 104 and a retard roller 105.
A double feed detection sensor 107 that generates an output for detecting double feed of originals is disposed between the feed roller 104 and the retard roller 105, and a registration roller pair 115.
In the conventional sheet feeding apparatus, generally when the double feed of originals is detected by the double feed detection sensor 107, the operation in which the originals picked up by the pickup roller 102 are separated one by one and fed by the feed roller 104 and the retard roller 105 is stopped, and a warning of double feed occurrence is issued. An operator feeds the double-fed originals again by hand based on the warning or the like, to prevent missing of images.
Thus, if the double feed of originals occurs during an image reading process, the sheet feeding is stopped, and it is necessary for the operator to remove the double-fed originals, and confirm whether or not any other originals being read remain in the apparatus.
Also, since the scanner itself temporarily stops operating each time the double feed of originals occurs, the operating rate is inevitably reduced. Thus, the working efficiency significantly decreases especially when a large number of originals are processed.
In order to solve such a disadvantage, there has been proposed a scanner which temporarily stops the feeding operation of originals when the double feed of originals is detected, automatically pulls back the double-fed originals 100a toward the sheet supply tray 101, and performs the separating and feeding operation again.
In such a scanner, after the double feed of originals is detected, the feed roller 104 is rotated in a direction opposite to a normal rotational direction (hereinafter simply referred to as “feeding direction”) to feed originals in the sheet feeding direction, and the retard roller 105 is rotated in a separating direction for separation of originals, so that the double-fed originals can be pulled back to the sheet supply tray 101 side.
A one-way clutch 501 is provided between the feed roller 104 and a driving unit, not shown, that drives the feed roller 104 in order to prevent the feed roller 104 from applying a load to conveyance while an original reaches a downstream convey roller to be conveyed.
Accordingly, even if the driving unit is reversely rotated for driving the feed roller 104 in the reverse direction, the driving torque is not transmitted via the one-way clutch 501 to the feed roller 104. Therefore only a conveying force transmitted from the retard roller 105 acts as a force to pull back the originals toward the sheet supply tray 101.
On the other hand, the retard roller 105 includes a torque transmitting member 502 such as a torque limiter or spring clutch for allowing a certain torque to act in the direction opposite to the feeding direction. The rotational driving force of a motor or the like is transmitted via the torque transmitting member 502.
Therefore, when the double-fed originals are pulled back to the sheet supply tray 101 side, the conveying force of the retard roller 105 may be limited by the slidable force of the torque limiter or spring clutch, and a sufficient reverse conveying force may not be obtained by the retard roller 105.
Accordingly, there has been proposed a technique in which a rotatable member having a protrusion is provided coaxially with the retard roller 105 and the rotatable member is rotationally driven in the direction opposite to the feeding direction, so that the protrusion pulls back the double-fed originals toward the sheet supply tray 101 (see Japanese Laid-Open Patent Publication (Kokai) No. 02-147533).
However, in this technique, the driving of the retard roller 105 and the driving of the rotatable member need to be separately controlled. In addition, since only the protrusion of the rotatable member contacts the originals, a sufficient reverse conveying force cannot be also obtained.
There has also been proposed a technique in which a feed roller 201 and a retard roller 202 are formed with comb-tooth-like shape grooves on the outer peripheral portions thereof, and are disposed such that the concave portions and convex portions of the feed roller 201 are facing respectively toward the convex portions and concave portions of the retard roller 202. And the feed roller 201 and the retard roller 202 are in non-contact with each other and overlap each other in perspective view of axial direction thereof as shown in FIGS. 11 and 12 (see Japanese Patent No. 3262064).
In this technique, the feed roller 201 is rotated in the feeding direction, and the retard roller 202 is rotated in the direction opposite to the feeding direction, so that the shaft of the feed roller 201 and the shaft of the retard roller 202 approach or move away from each other as shown in FIG. 12. The overlapping amount between the concave portions and convex portions is thereby changed, and the originals picked up by the pickup roller 102 in the feeding direction are separated one by one and fed.
In this case, the retard roller 202 is constantly rotated in a separating direction to pull back the originals oppositely from the feeding direction during the separating and feeding operation, and a rotational torque for rotating the retard roller 202 in the direction opposite to the feeding direction can be sufficiently ensured. However, in the case of such a configuration, a sufficient feeding force for originals cannot be obtained since the comb-tooth-like feed roller 201 and the comb-tooth-like retard roller 202 are in non-contact with each other as described above. Thus, the pickup roller 102 supplements the feeding force for originals separated and fed by the feed roller 201 and the retard roller 202.
On the other hand, when the double-fed originals are conveyed in a reverse direction to pull back the originals toward the sheet supply tray 101 by the feed roller 201 and the retard roller 202 after detecting the double feed of originals, a sufficient reverse conveying force cannot be obtained since the pickup roller 102 is retracted.
Also, there has been proposed a technique in which a feed roller 104 and a separating pad member 301 are used to separate and feed originals as shown in FIG. 13.
In this technique, even when the feed roller 104 is to be reversely rotated for pulling back the originals toward the sheet supply tray 101 after detecting the double feed of originals, the feed roller 104 cannot be reversely rotated since a one-way clutch or the like is incorporated in the feed roller 104 in most cases.
Also, an envelope-shaped original or an original with a sticky note being attached thereto may be read by an image reading unit 116.
When double feed detection is performed with respect to the original having an overlapping portion as described above by using the double feed detection sensor 107 that is singly provided on the conveying pass as shown in FIG. 14, double feed is detected even when the original is normally conveyed. Therefore, at the time of conveying the original with a sticky note being attached thereto, for example, it is necessary to stop detection of the double feed of originals in a position where the sticky note is attached.
In order to solve this problem, an apparatus having a plurality of double feed detection sensors 107 disposed in the width direction of the conveying pass has been proposed as shown in FIG. 15.
In such an apparatus, an area in which the double feed is detected is set in advance, and the plurality of double feed detection sensors 107 are disposed in the width direction of the conveying pass. Accordingly, even if the sticky note is attached to the original, the detection of double feed of originals is stopped in the position where the sticky note is attached.
Also, the double feed of originals can be detected in a plurality of positions where the double feed detection sensors 107 are arranged. Therefore, even when originals having difference sizes are mixedly stacked on the sheet supply tray 101, the double feed of originals can be detected.
For example, as shown in FIG. 15, in a case where an small size original 100a slips into large size originals 100b and the originals are double-fed with the small original 100a being decentered to one side, the double feed can be detected. Missing of images or a paper jam can be thereby prevented.
However, in the case of such double feed of originals, at the time of pulling back the double-fed originals toward the sheet supply tray 101 by the feed roller 104 and the retard roller 105 after detecting the double feed of originals, the feed roller 104 and the retard roller 105 may not contact the double-fed small original.
If the operation of pulling back the originals is performed in such a state, the small original on the large original is taken by the large original to be pulled back toward the sheet supply tray 101 when the large original in contact with the feed roller 104 and the retard roller 105 is pulled back toward the sheet supply tray 101.
In this case, if the small original is prevented from moving toward the sheet supply tray 101 by contact with a conveying guide or the like, the small original is possibly left on the conveying pass. If the sheet feeding is resumed in this state, a paper jam or original breakage may be caused.